Protein citrullination is increased in virtually all inflammatory diseases, as well as cancer, and the evidence indicates that this post-translational modification is a major contributor to disease pathogenesis. In fact, inhibition of protein citrullination decreases disease severity in animal models of rheumatoid arthritis, ulcerative colitis, nerve damage, and cancer. Thus, the PADs, which catalyze this PTM, represent novel therapeutic targets. The overall goal of this proposal is to build upon the PI's exciting research program that has explored PAD biology, biochemistry, and inhibition to further validate the PADs as therapeutic targets for a range of diseases. Additionally, the PI seeks to identify novel citrullinated biomarkers of disease. The identification of citrullinated biomarkers will allow for better, more tailored treatments, because they can be used to diagnose and monitor the efficacy of treatments for these diseases. Specific questions to be addressed during the project period include: (1) How are the PADs activated in cells?; (2) What proteins are PAD substrates and how does this modification effect their activity?; (3) Can citrullinated proteins be used as biomarkers of disease?; and (4) Can isozyme specific inhibitors and activity based proteomic probes be generated for the PADs?. By addressing each of these questions, the PI expects to provide a holistic understanding for how the PADs contribute to human biology. Given the outstanding track record of the PI and his leadership position in the PAD field, combined with his strong network of collaborators, this project will provide sorely needed research tools to the scientific community and will undoubtedly result in the discovery of new PAD biology that is predicted to have major translational impact on autoimmunity and cancer.